Wind power is considered one of the cleanest, most environmentally friendly energy sources presently available, and wind turbines have gained increased attention in this regard. A modern wind turbine typically includes a tower, generator, gearbox, nacelle, and one or more rotor blades. The rotor blades capture kinetic energy of wind using known airfoil principles. For example, rotor blades typically have the cross-sectional profile of an airfoil such that, during operation, air flows over the blade producing a pressure difference between the sides. Consequently, a lift force, which is directed from a pressure side towards a suction side, acts on the blade. The lift force generates torque on the main rotor shaft, which is geared to a generator for producing electricity.
During operation, wind impacts the rotor blades and the blades transform wind energy into a mechanical rotational torque that rotatably drives a low-speed shaft. The low-speed shaft is configured to drive the gearbox that subsequently steps up the low rotational speed of the low-speed shaft to drive a high-speed shaft at an increased rotational speed. The high-speed shaft is generally rotatably coupled to a generator so as to rotatably drive a generator rotor. As such, a rotating magnetic field may be induced by the generator rotor and a voltage may be induced within a generator stator that is magnetically coupled to the generator rotor. The associated electrical power can be transmitted to a main transformer that is typically connected to a power grid via a grid breaker. Thus, the main transformer steps up the voltage amplitude of the electrical power such that the transformed electrical power may be further transmitted to the power grid.
In many wind turbines, the generator may be electrically coupled to a bi-directional power converter that includes a rotor-side converter joined to a line-side converter via a regulated DC link. Further, wind turbine power systems may include a variety of generator types, including but not limited to a doubly-fed asynchronous generator (DFAG).
DFAG operation is typically characterized in that the rotor circuit is supplied with current from a current-regulated power converter. As such, the power converter can provide nearly instantaneous regulation of its output currents with respect to the grid frequency. Under steady operating conditions, the rotor-side converter controls the magnitude and phase of currents in the rotor circuit to achieve desired values of electromagnetic torque. Reactive power flow into the line-connected stator terminals of the generator can also be controlled.
A simplified, schematic diagram of one embodiment of a main circuit 10 of a DFAG is illustrated in FIG. 1. As shown, the main circuit 10 includes a generator 12 connected to a power converter 14 (as well as any required power electronics) and a transformer 16 that is connected to a power grid 18. More specifically, as shown, the connection to the power grid 18 is at the high side of the transformer 16, where the voltage and current are indicated as VG and IG, respectively. Further, the power grid 18 is illustrated conceptually as a Thevenin (Th) equivalent. As used herein, the Thevenin equivalent generally refers to an equivalent voltage source in series connection with an impedance that is an approximation of a complex, non-linear grid that constantly changes based on the number of wind turbines, external grid status, etc. The Thevenin impedance ZTh varies with the number of wind turbines in operation and the status of the transmission system beyond the wind park.
When an imbalance occurs in the power grid 18, a negative-sequence component of voltage appears in the Thevenin voltage, which is represented in FIG. 1 as VTh. Such a negative sequence component of voltage can have a negative impact on the power grid 18 as well as the wind turbine power system. As such, it would be advantageous to provide a control methodology for reacting to this negative sequence component of voltage so as to offset any negative impacts.
Prior art systems often design the control system to attenuate the negative sequence component of current flowing in the equipment as a result of the imbalance in the power grid 18. While this reduces the duty on the equipment, it is not necessarily the best for the power system to which it is connected. As such, an improved system and method for providing negative-sequence control that causes the wind turbine to follow a prescribed relationship between the voltage VG and the current IG would be advantageous. For example, a negative-sequence control scheme that causes the relationship between the voltage VG and the current IG to be inductive nature, similar to how a directly-connected synchronous machine would react, would be desirable.
Thus, the present disclosure is directed to controlling a negative sequence current in the DFAG of the wind turbine that addresses the aforementioned issues.